The present invention relates to transparent and colorable elastomeric compositions and, more particularly, to transparent and colorable elastomeric compositions that can be used in reinforcing applications.
Rubber compositions are used in a variety of applications, including tire components such as treads and sidewalls, hoses, belts, footwear components, vibration isolation devices and bladders. While the particular rubber compositions used in each of these applications vary widely in their physical properties, one attribute remains the samexe2x80x94their color. Most rubber compositions are black. Furthermore, most rubber compositions eventually become discolored due to heat, light, ozone, etc. This is particularly true for rubbers used in stressful, demanding applications such as tire treads and sidewalls.
Practitioners in this field will point to the presence of the reinforcing filler xe2x80x9ccarbon blackxe2x80x9d as a prime reason that most rubbers are black. While this is true, carbon black is not the only factor. In fact, a wide variety of other fillers, curatives, antidegradants, oils and the rubbers themselves can all result in a dark color that is essentially impossible to pigment. This is evident in compositions where carbon black has been replaced with a silica filler and the rubber is still discolored. For example, European Patent 0 682 071 B1 discloses a silica reinforced tire tread which, due to the presence of the aromatic processing oil, coupling agent, antidegradants and a sulfur curative system, will still be dark in color. In fact, it is uncertain how many of the ingredients present in the rubber composition would have to be changed to produce a colorable composition.
Of course, some colorable and transparent elastomeric compositions do exist. For example, clear EPDM elastomers are available. However, these elastomers do not covulcanize with other rubbers. Since many rubber applications involve combining several types of rubber to form a single article (i.e. tires), these EPDM elastomers are limited in their usefulness.
White sidewalls on tires are a form of colorable rubber. The white color is achieved by using fillers such as silica, clay, talc and carbonates instead of carbon black and adding titanium dioxide as a whitening pigment. However, the white color comes with a price. The fillers are more fragile than carbon black and result in a weak rubber composition that does not reinforce the tire. Therefore, the rubbers used for white sidewalls are also limited in their usefulness.
The present invention provides improved transparent and colorable elastomeric compositions. The transparent elastomeric compositions can be covulcanized with rubbers such as polybutadiene, polyisoprene, styrene-butadiene rubber, styrene-isoprene-butadiene rubber, isoprene-butadiene rubber, ethylene-propylene diene rubber or natural rubber. The colorable rubber compositions have sufficient properties to function as a reinforcing member in an automobile tire. Preferably, both the transparent and colorable elastomeric compositions include at least one copolymer of a C4 to C7 isoolefin and a para-alkylstyrene, silica and a coupling agent.
The elastomeric compositions of the present invention are useful in a variety of applications, particularly pneumatic tire components, hoses, belts, solid tires, footwear components, rollers for graphic arts applications, vibration isolation devices, pharmaceutical devices, adhesives, sealants, protective coatings and bladders for fluid retention and curing purposes.